The principle of protecting electric motors using bimetal switches such as that shown in British Pat. No. 903807 is well known. The protector switch is series connected with the motor windings and is required to open circuit when the motor is overloaded before the windings reach their temperature limit, and yet must allow the full load current to pass at all ambient temperatures experienced by the motor in order that the motor be able to develop its full designed load. This is particularly difficult in the case of permanent magnet low voltage motors as used for example in motor vehicles. In addition to providing a large torque from a small package, such motors are also required to perform satisfactorily and be protected in ambient temperatures from -40.degree. C. to +80.degree. C. in order to meet the car manufacturer's specifications.
In addition to having to operate through such a wide range of ambient temperatures, such motors must also be constructed in a light weight manner in order to reduce the weight of the vehicle and improve its fuel consumption. The light weight construction means that the motor has a low thermal mass and its temperature rises quickly when it is loaded, and yet it must be capable of producing the full torque specified by the vehicle manufacturer. Motor protector devices are therefore subject to a number of conflicting requirements in their performance characteristics; the development of large torques requires large motor currents to be sustained for short periods without nuisance tripping of the protector, and yet the protector must protect against the motor stall current which decreases with time because the associated high winding temperatures cause an increase in the resistance of the winding. A typical automotive window lift motor, for example, is required to work at a high load for a short period (e.g. 6 seconds) with currents close to a 20 ampere stall current without the protector nuisance tripping. The current heating of the armature under stall conditions leads to an increase in the temperature of the windings, and a decrease in the stall current to approx. 12 amperes It can thus be seen that the protector must be relatively insensitive to large currents, yet be sensitive to low currents over an extended period.
Devices such as that shown in British Pat. No. 903807 are used extensively in the protection of low voltage electric motors. The characteristics of these devices can be shown by plotting the break time with varying currents. FIG. 1 of the accompanying drawings in which the vertical axis represents current measured in amperes and the horizontal axis represents break time in seconds, shows that the characteristic for prior art devices of the type of British Pat. No. 903807 as shown by the continuous lines in FIG. 1 is relatively flat, and this leads to a requirement for a number of alternative switches having different bimetal resistances so as to provide the required characteristic at the high current or low current end. It is frequently difficult to manufacture a switch having the required characteristics at both ends of the time scale, and commonly the protector utilized is a compromise between both requirements. Nevertheless, protectors of this type have provided adequate protection for motors constructed in traditional ways.
The design of motors of even lighter construction has required the characteristics of such protector switches to be changed in order to give longer break times on high currents and to lower the value of the current at which the protector will ultimately trip. One approach to this requirement has been to manufacture the protector from two pieces of bimetal mounted on a chassis. A thinner bimetal is mounted back to back with a thicker bimetal and is used to give an initial increase in contact pressure between a movable contact on the thicker bimetal and a fixed contact on the chassis in order to raise the operating temperature of the protector and provide a longer break time under large currents. The opposite action occurs under falling temperature and the remake time is also increased, thus ensuring that the energy input to the motor under stall is limited so that the windings do not exceed a safe working temperature. Protectors of this construction are slow make and break protectors and require large silver contacts in order to withstand the arcing action; this arcing action also leads to radio interference on car radios, etc., and may influence the working of electronic devices on the car. The manufacture of the devices is complicated and the performance characteristics can vary considerably with life as the silver contacts are worn by the arcing, causing changes in the geometry of the bimetal.
Other known devices use bimetal with separate heater bridges or an internal or external heater in order to modify the device characteristics. The performance of such protectors, however, varies with the orientation of the protector, and they have a complicated construction in order to transfer the heat from the heating member to the bimetal.
Some of the disadvantages of the device shown in British Pat. No. 903807 have been minimized by a method described in German Patent Application No. P2811503.9-32. In this application, a bimetal carrying terminal of the protector device is attached to a metal brush holder of the electric motor or to the thick wire windings of an interference suppressor. During a short period of high current, the comparatively cool metal brush holder or windings acts as a heat sink and reduces the rate of temperature rise of the bimetal in the device so as to prolong the first break time. In the case of a prolonged stall or overload, the metal brush holder or windings heat up and heat then flows from the brush holder or winding into the bimetal via the brass terminal, and this helps the bimetal to operate under a lower current. The heat input also slows down the cooling of the bimetal; this leads to an extended remake time and lower energy input into the motor with consequently lower winding temperatures. This method clearly can only be applied to motors having suppressors or comparatively substantial metal parts in the brush holder construction.